Dump trucks and wagons and other dumpable vehicles are widely used throughout industry and on farms and come in a wide variety of types. Generally, they include a dump bed having a dump chassis pivotally connected to a main chassis, usually a subframe or the vehicle chassis, and operated by a hydraulic hoist mechanism extending between the dump chassis and the main chassis. Two of the most popular types of hoists currently used are double-arm hoists and scissors hoists.
An illustrative prior art double arm hoist is shown in FIG. 1A of the accompanying drawings. It includes a vehicle chassis equipped with wheels and tires and with a sub-frame (main chassis) atop the vehicle chassis. A tilted dump chassis is attached by a dump hinge to the rear (left end) of the sub-frame. The remainder of the dump bed is omitted from the figure.
The hoist proper is shown in open or elevated position and has a pair of triangular arms, hence the term "double-arm hoist". These arms have their lower ends attached for pivotal motion about a pivot shaft secured crosswise in the sub-frame, and their upper ends are equipped with rollers adapted to roll in captive relationship in longitudinal channels in the dump chassis longbeams.
The hoist reaches the position shown in the drawing by extension of a hydraulic cylinder connected between a cross-member of the subframe and a cross-shaft joining the mid-portions of the triangular arms. Such extension causes those arms to rotate counter-clockwise about the pivot shaft from a horizontal rest position and to lift on the longbeams as the rollers roll rearwardly in the channels toward the dump hinge. Another example of a double-arm hoist would have pivoted lift links in lieu of the rollers. A wide variety of hoists of these general types have been offered commercially for many years.
FIG. 1B illustrates a prior art scissors hoist in open or elevated position. Here, the tilted dump chassis is directly connected by a dump hinge to the rear of the vehicle chassis.
Included in this hoist are two laterally spaced apart and similar pairs of upper and lower arms located to the left and right sides of the longitudinal center line of the vehicle. One set is hidden behind the other in this right-side view. Both sets normally operate in unison and in the same manner on their respective sides.
Through a main pivot axis at their overlapping ends and through connections at the upper end of the upper arm and at the lower end of the lower arm, respectively, the upper and lower arms in each set are pivotally connected to each other and to the vehicle and dump chassis. The two sets of arms open and close by pivotal scissors motion about their common main pivot axis, which may be a transverse shaft extending between the two sets of arms.
Due to the "L" shape of the arms, the main pivot axis lies well below the elevations of the dump hinge and of the connections between the arms and the respective chassis, when the scissors are closed. Expansion of a hydraulic cylinder located between and connected with both of the sets of scissors arms opens the hoist and tilts the dump chassis.
There is a difference in the arcs travelled by the upper arms and by the portions of the dump chassis to which the arms are attached at the beginning of the dump cycle. Due to the geometry of the arms, this difference could cause binding or straining of the hoist. This difficulty exists only at "breakaway", e.g. during the first few degrees of lifting of the dump chassis (e.g. about 2-3 degrees), which may be alternatively expressed in terms of the percentage of the total stroke of the hydraulic cylinder, e.g. about 5 percent or less, utilized in attaining these few degrees of lift. This arc difference is readily accommodated and the binding/straining problems are readily overcome by connecting the upper arms to the dump chassis through a very short-throw lost motion device discussed in greater detail in U.S. Pat. No. 3,791,695. See also British Patent No. 2,049,566. Substantial numbers of scissors hoists have been sold and utilized commercially.
Each of these types of hoists has its respective advantages as well as its disadvantages. Double-arm hoists generally out-perform scissors hoists in delivering high torque at the breakaway point, i.e. when the dump bed first starts to lift. However, as the dump angle increases, the amount of torque that a double arm hoist can apply to the bed and its payload decreases. Another disadvantage of the double-arm hoist is that it must be located further toward the back of the truck chassis than the scissors hoist in order for its hydraulic cylinder arm to push the truck bed to its maximum dump angle. When the cylinder is near the back of the truck, excessive stress is placed on the rear hinge and hoist operation is less efficient.
A scissors hoist of comparable size has an opposite advantage and disadvantage. For example, a scissors hoist usually provides relatively low torque at the breakaway point. On the other hand, available torque increases as the dump angle increases, up to a certain dump angle. Another advantage of a scissors hoist is that it can be located on the chassis further towards the front of the truck than a double-arm hoist, thus avoiding undue stress on the dump bed pivot connection.
Consideration of the opposite advantages and disadvantages of double-arm hoists and scissors hoists led me to pursue the development of a hoist which would combine the advantages of these two types of hoists while mitigating their disadvantages. My discovery of how to accomplish this objective was disclosed in U.S. Pat. No. 4,302,050 (the '050 patent), issued Nov. 24, 1981 and entitled "TRUCK HOIST".
The hoist of the '050 patent utilizes a pivoting stop means and two rotatable pairs of arms to provide a hoist assembly which acts as a double arm hoist during the initial lifting action, from the time the dump bed lifts off the chassis to the time when the dump bed is at a predetermined "switching angle" with the chassis, and as a scissors hoist during continuation of the lifting action from the switching angle to the uppermost dump angle. This hoist assembly has a lifting capacity which exceeds that of a double-arm hoist or scissors hoist of comparable size as shown by FIG. 11 of the '050 patent and as discussed therein. Moreover, this hoist assembly has performed superbly in the field.
Competition in the field provides continuing pressure for development of hoists with even greater lifting capacity and hydraulic force efficiency. It is the purpose of this invention to fulfill this need and other needs which will become apparent to the skilled artisan upon becoming familiar with the following disclosure.